When a centrifuge or similar apparatus is implemented to separate a mother liquid from solids, it is desired in a multitude of processes to wash the solids retained in the centrifuge or similar apparatus to remove either the vestiges remaining of the mother liquid and/or to purify the solids retained. This is achieved by introducing another liquid, a “wash liquid”, into the centrifuge after a quantity of the mother liquid has been removed by the centrifuge.
In using a centrifuge or similar apparatus in a process in which the solids are soluble in the wash liquid (referred to herein as Class X), the improved purification of the solids must be offset by the loss of the dissolved solids, the reduction in separation efficiency and the necessary process to separate the wash liquid from the separated mother liquid and recover any dissolved solids. An example of this situation is that of separating massecuite into sugar crystals and molasses. Whilst retaining the sugar crystals in the basket, the separated molasses and wash liquid require a further separation with minimal intermixing, to be processed in separate streams, the wash liquid being applied after the bulk of the molasses has been separated to wash the crystals to the required purity levels.
In a process involving a centrifuge or similar apparatus in which the solids are insoluble in a wash liquid (referred to herein as Class Y), the solids, after separation, may be washed to remove further mother liquid from its surfaces. The extent of the wash must be offset against the additional loading of the further separation stage that must be provided to remove the contaminants from the excess liquid used to wash the solids. An example of this situation is that of producing gypsum in flue gas desulphurisation processes. Washing during the centrifuge part of this process reduces the chloride contamination of the solids to produce high grade gypsum suitable for wall board manufacture. The mother and the wash liquids are mixed and reprocessed as an effluent.
The use of wash liquid in excess of the minimum required is known as “overwashing”. Overwashing is detrimental to the separation process and results in reduced separating efficiency, increased process cycle times, excess wash liquid usage, excess dissolution of solids, increased load on secondary effluent separating process or combinations of these.
Thus, the amount of wash liquid used affects the efficiency and economy of implementing a centrifuge.
It is known to seek to control overwashing by monitoring the liquid state as it leaves the centrifuge case. FIG. 1 of the accompanying drawings shows a typical industrial centrifuge comprising a perforated cylindrical basket/drum 10 which has a perforated outer surface and is rotatable about a vertical axis 12 on a motor driven shaft 14. The perforated basket 10 has a screen 15 on its cylindrical inner surface and is contained within a cylindrical outer casing 16 having an outlet pipe 18 at its lower end for leading off liquids centrifugally separated from solids 20. A pipe 22 enables a wash liquid to be sprayed onto the solids 20 in the basket retained by the screen 15.
A measurement of the state of the wash liquid is made at a measurement location 19 in the outlet pipe 18.
The flow of wash liquid through the centrifuge—from the stationary wash pipe 22 to the rotating basket 10, through the solids 20, basket perforations and screen 15 to flow down the stationary casing 16 inner surface 24 to the casing outlet 18—is complex. It varies with the liquid viscosities, screen type, basket perforations pattern, casing dimensions, centrifugal speed, windage and outlet position, all of which affect the flow rate. Of concern here is the liquid flow as it leaves the rotating basket and spirals down the inner surface 24 of the casing 16.
In an industrial centrifuge, the time period for the wash liquid to reach the outlet pipe from the basket perforations is typically between 5 and 30 seconds. Thus any measurement of the state of the wash liquid immediately after the point of contact with the solids will be delayed by at least this time during which overwashing may have occurred. Thus a flow time of 20 seconds from perforations/screen to the outlet to provide a minimum (ideal) solids wash time of 20 seconds requires 40 seconds total wash time and results in a 100% overwashing. These weaknesses are most marked on large centrifuges processing viscous liquids.
If the flow of the wash liquid is set at a fixed time to ensure a full wash under idealized conditions of maximum process throughput and minimum available wash liquid flow rate, then further overwashing will occur as the process parameters vary from the ideal.
Overwashing, a weakness of all known existing systems of wash liquid control, is detrimental to the separation process and, depending on the application, may result in one or more of:
(a) reduced separation efficiency,
(b) increased process cycle times,
(c) excess wash liquid usage,
(d) excess dissolution of solids and
(e) increased load on secondary effluent separating processes.
Thus, the present state of the art measuring the liquid condition at the outlet (18) requires the full flow of the liquid at the outlet pipe measuring point (19), and gives the required measurement signal only after the liquid has traveled from the perforations/screen to the outlet, a delay ranging from 5 to 30 seconds. Setting a fixed wash time of flow for a correct wash at maximum basket fill level and minimum wash flow rate results in overwashing on all throughputs including the maximum. These weaknesses will be most marked on large centrifuges processing viscous liquids (e.g. in Class X, sugar losses of 10% of the factory sugar output have been recorded by overwashing during centrifuging with fixed time wash control).